"subscriptions:insert" returns an error

Today I noticed that the subscriptions:insert method of YouTube Data API V3 returns this error:

{
  "error": {
    "errors": [
      {
        "domain": "youtube.subscription",
        "reason": "publisherNotFound",
        "message": "The resource specified by the requests <code>snippet.resourceId</code> property cannot be found."
      }
    ],
    "code": 404,
    "message": "The resource specified by the requests <code>snippet.resourceId</code> property cannot be found."
  }
}

This is the request body that I sent:

{
  "snippet": {
    "resourceId": {
      "channelId": "UCa90xqK2odw1KV5wHU9WRhg"
    }
  }
}

The above request was valid before. But now, it seems that we need a kind property for snippet.resourceId see below:

{
  "snippet": {
    "resourceId": {
      "channelId": "UCa90xqK2odw1KV5wHU9WRhg",
      "kind": "youtube#channel"
    }
  }
}

It would appear that subscribing to channels without this step is regarded as an invalid request.